1. Field of the Invention
The present invention relates in general to a line sensor and a radiation-data readout apparatus, and in particular to a line sensor provided with a photodiode that serves as a photoelectric converting element and a radiation-image data readout apparatus.
2. Description of the Related Art
There are known line sensors wherein the light-receiving portion thereof comprises a plurality of photodiodes serving as photoelectric converting elements, which are arranged in a row and with a predetermined interval between each photodiode. In these line sensors, as shown in the side view of the line sensor depicted in FIG. 1A, the light-receiving surface (the shaded portion in FIG. 1A) of each photodiode 1, generates a quantity of electrical charge corresponding to the quantity of light energy received thereon. These charges are read out through a readout gate 2 in the direction of a charge transmitting portion 3 such as a CCD or the like, and are again transmitted by the transmitting device 3 to an external portion; such line sensors are widely applied in a number of fields as apparatuses for detecting light. For example, there are radiation-image data readout apparatuses for reading out the radiation-image data that has been recorded on a stimulable phosphor sheet and which comprise a built-in line sensor for detecting radiation.
The aforementioned stimulable phosphor sheet utilizes a layer containing stimulable phosphors (storable phosphors) formed on a substrate, which upon the irradiation thereof with radiation store a portion of the radiation energy thereof, and emit upon the subsequent irradiation thereof with a visible light, a laser light or other excitation light, a stimulated emission corresponding to the radiation energy stored therein. These stimulable phosphor sheets are widely used in conjunction with radiation-image recording and reproducing systems, wherein: the radiation-image data of a human body or other subject of photographing through which radiation has been passed is temporarily stored on a stimulable phosphor sheet; said stimulated phosphor sheet is scanned with a laser or other excitation light causing a stimulated emission to be emitted therefrom; said stimulated emission is photoelectrically read out sequentially by a photoelectric readout means to obtain an image signal and; on the other hand, said stimulable phosphor sheet is then irradiated with an erasing-light to erase the radiation energy remaining thereon (refer to Japanese Unexamined Patent Publication Nos. 55(1980)-12429, 55(1980)-116340, 56(1981)-104645, etc.).
Here, according to a radiation-image data readout apparatus to be employed in the above-described radiation-image recording and reproducing system: a line light source is used as the excitation light source for irradiating the sheet with an excitation light in a line form; a line sensor provided with a plurality of photodiodes arranged in a line along the lengthwise direction (herein after referred to as the main scanning direction) of the line-form portions of the sheet which have been irradiated with the excitation light by the line light source is employed as a photoelectric readout means; and a scanning means is provided for moving the line light source and the line sensor relatively from one end to the other of the sheet in the direction substantially perpendicular to the main scanning direction (hereinafter referred to as the sub-scanning direction) of the sheet; wherein the aim of this configuration is to obtain a shortening of the readout time and a reduction in the size and cost of the apparatus, have been proposed (refer to Japanese Unexamined Patent Publication Nos. 60(1985)-111568, 60(1985)-236354, 1(1988)-101540, etc.).
Incidentally, due to the fact that the potential well of the internal portion of the photodiode 1 of a conventional line sensor is shallower than that of the potential well of the lower portion of the readout gate 2, although read out of the charge is realized, the potential of the internal portion of the photodiode is the same. FIG. 1B shows the configuration of the potential along the line A-B of the line sensor shown in FIG. 1A. In order to raise the efficiency of photodetection, for cases in which a large photodiode is employed, that is, for cases in which the width occurring in the direction Y of the shaded portion shown in FIG. 1A is broad, because it takes an long period of time until the charge generated at a portion remote from the readout gate 2 reaches the readout gate 2, complete readout thereof is difficult, and the charge readout efficiency is poor. Therefore, a problem arises in that in order to completely readout the full charge, increases in the readout voltage and readout time cannot be avoided.
This problem is conspicuous in the radiation-image data readout apparatus described above as well. Because the stimulated emission emitted from the stimulable phosphor sheet is extremely faint, it is desirable that as much of the stimulated emission as possible is received; therefore, it is necessary to enlarge the size of the light-receiving area of the light-receiving portion, that is, to enlarge the dimension of each photodiode in the direction (in FIG. 1A, the direction indicated by the arrow Y) perpendicularly intersecting the direction in which the rows of photodiodes extend.